1. Field of the Invention
The present invention relates to subsurface well completion equipment and, more particularly, to methods and related apparatus for the metering and control of fluids that have been separated by downhole apparatus.
2. Description of the Related Art
Commonly, in subterranean oil producing formations, oil and water coexist in varying ratios. The most commercially ideal situation for an oil producing company is to have a well where the percentage of water, commonly referred to as the "water cut" to be as close to zero as is practical, but in reality as oil is produced from the formation, the water cut percentage invariably increases. Water produced with the oil is a problem for operating companies since it must be separated from the oil at the earliest possible point in the oil production process to avoid the costs associated with handling and or transporting and disposing of large volumes of water. This is especially true in wells with a high water cut, wherein the percentage of water is 75% or greater.
In the past, equipment has been used to separate the oil from the water after it has been lifted to the surface. The most basic method is to allow the produced fluid to flow into a large tank for "settling". The difference in density of the two fluids causes a separation to occur. The water is removed from the bottom of the tank and is disposed of, leaving the crude oil for use by the operator. A third product, dissolved gas which breaks out of solution as a result of decreased pressure, must also be managed by the surface equipment. This method is very slow and expensive. Over time, smaller separators were developed that allowed portions of the water cut and gas to be removed from the oil at the surface, but the expense of lifting the water to the surface, and the disposal thereof, still represented a significant expense. In these cases the separated oil is moved to storage tanks until transport by pipeline, truck or tanker is arranged. The water is disposed of, generally by being reinjected into the original formation, or in a disposal well. In the case of high water cut wells, the volume of water handled can be 80 to 90 percent of the total production of the well. The ultimate economics of the well dictate that when the cost of lifting and disposal of water exceed the value of the crude oil being produced, the well must be abandoned, leaving valuable crude oil in the formation.
More recently, methods have been developed to separate the oil from the water downhole, either by filtration, as described in U.S. Pat. No. 4,241,787, or by centrifugal force in devices well known to those skilled in the art called "hydrocyclones". Hydrocyclones positioned deep in the well and used in conjunction with downhole electric submersible pumps (commonly called ESP's) separate the oil and water by taking advantage of the difference in the density of the two fluids. In application, the oil/water mixture is pumped tangentially and rotationally into a cylindrical chamber in the hydrocyclone causing a separation vortex. Centrifugal force in the vortex causes the fluids to separate, with the water passing out the bottom of the hydrocyclone, and the oil passing out the top. The resultant oil portion can be lifted to the surface, while the water portion can be reinjected directly into the formation from whence it was produced, or it can be routed into a disposal stratum. The hydrocyclones can be arranged in a series to increase the efficiency of the device to relatively match the water cut. The advantages of downhole separation of the produced oil/water mixture are obvious. The excess water does not have to lifted to the surface, solution gas remains dissolved in the water and is distributed with the water in the disposal stratum, and the surface separation facilities can be much smaller and less expensive. The end is enhanced economics of the produced well resulting in a greater percentage of oil being recovered from the formation.
In order for downhole hydrocyclones to function optimally, controlled back pressure must be maintained, as small pressure fluctuations on the discharge radically effect the efficiency thereof. When the water cut portion of the produced fluid is not lifted to the surface where it can be directly measured, the operator has no direct indication of the water cut percentage, and how it changes over time. This leads to a decreased ability to manage the reservoir and to monitor the efficiency of the separation hydrocyclones.
There is a need for an apparatus to enhance and optimize the operation of downhole separation hydrocyclones by metering and controlling fluids being reinjected into the formation by: providing a back pressure on the hydrocyclone; controlling the water injection flow rate; monitoring the total volume of fluid injected into the formation; and monitoring temperature, as well as providing an indication of up and downstream pressure on the apparatus. There is also a need for an apparatus to similarly monitor the fluid volume being lifted from the well to the surface.